So I have some lemon balm and peppermint in my garden and we use it to make tea.

I really like the smell and aroma of lemon balm and if I look online, it mentions for example that lemon balm is used for relieving anxiety, stress and can improve cognitive function.

But how is this tested, how can people even tell?
Does it really have this effect? And what would happen if you drink a lot of it?